In recent years, liquid crystal display devices, which have spread rapidly to take the place of cathode-ray tubes (CRTs), have been widely used in liquid crystal televisions, monitors, mobile phones, and the like thanks to their energy-saving, low-profile, lightweight features, etc. An example of a method for further exploiting these features is to improve an illumination device (so-called backlights) that is disposed in the back of a liquid crystal display device.
Illumination devices are classified broadly into side-light backlights (referred to also as “edge-light backlights”) and direct backlights. A side-light backlight is configured to have a light guide plate provided behind a liquid crystal display panel and a light source provided on an end face (lateral end) of the light guide plate. A beam of light emitted from the light source is reflected by the light guide plate to uniformly illuminate the liquid crystal display panel indirectly. This construction makes it possible to realize an illumination device low in luminance but capable of being made thinner and excellent in uniformity of luminance. Therefore, side-light illumination devices are employed mainly in small-to-medium-sized liquid crystal displays such as those used in mobile phones and laptop computers.
Meanwhile, a direct illumination device has a plurality of light sources arranged behind a liquid crystal display panel to illuminate the liquid crystal display panel directly. Therefore, direct illumination devices are employed mainly in large-sized liquid crystal displays of 20 inches or larger. However, the existing direct illumination devices are as thick as approximately 20 mm to 40 mm, thus hindering further reductions in thickness of displays.
Accordingly, an attempt has been made to reduce the thickness of a large-size liquid crystal display by arranging a plurality of side-light illumination devices (e.g., see Patent Literatures 1 and 2).
Each of the illumination devices (surface light source devices) described in Patent Literatures 1 and 2 has light guide plates, which is plate-like light guide blocks, joined together one after another along the direction of primary light (longitudinal direction), thereby having a tandem construction including primary light sources that supply primary light to their respective light guide blocks. Such an illumination device configured by arranging a plurality of light-emitting units (light source units) each constituted by combining a light source and a light guide plate is referred to generally as “tandem illumination device”.